A push switch providing a light click feeling has been often used for an input/operation section of various electronic devices. A push switch is demanded to be activated with a light operation force to reduce a burden to fingers and to have a long operation stroke particularly for use as an input device, such as a mouse, of a personal computer.
FIGS. 6 and 7 are a cross-sectional view and an exploded perspective view of conventional push switch 501 disclosed in Japanese Patent Laid-Open Publication No. 9-147666. Lower case 1 has a box shape and is made of insulating resin and opens at its upper side. Lower case 1 has an recess having a bottom. Inner fixed contact 2A and outer fixed contacts 3A and 3B are exposed from the bottom surface and are fixed by an insert molding. Inner fixed contact 2A is fixed at the center of the bottom. Outer fixed contacts 3A and 3B are located at an outer periphery of the bottom. Inner fixed contact 2A is located at the midpoint between outer fixed contacts 3A and 3B. Outer fixed contacts 3A and 3B are electrically independent from inner fixed contact 2. Inner fixed contact 2A connected to terminal 2 for connection to an external circuit extends outwardly from side wall 1A of lower case 1. Outer fixed contacts 3A and 3B connected to terminal 3 for connection to an external circuit extend outwardly from side wall 1A.
Movable contact 4 is made of thin plate if conductive and elastic metal and is shaped to have a circular dome shape having an opening at its lower part. Edges of the outer peripheral end of movable contact 4 are located on outer fixed contacts 3A and 3B, respectively. The deepest point of the dome shape of the lower surface of movable contact 4 faces inner fixed contact 2A by a distance.
Elastic operation body 5 is located on movable contact 4. Elastic operation body 5 has flange section 5A having a circular disk shape, upper projection 5B projecting from the center of an upper surface of flange section 5A, lower projection 5C projecting from the center of an upper surface of flange section 5A, and tubular section 5D protruding from an outer peripheral edge of flange section 5A. Lower projection 5C has a lower end contacting an upper surface of the top of the dome shape of movable contact 4.
Push button 6 is made of insulating resin and has columnar section 6A and flange 6B provided at the lower part of columnar section 6A. Columnar section 6A has a lower surface having recess 6C provided at the center thereof. Push button 6 is provided on elastic operation body 5, such that the upper part of upper projection 5B of elastic operation body 5 is inserted in recess 6C.
Upper case 7 made of insulating resin is attached to lower case 1 by being caulked with projection 1B of lower case 1 to cover the recess of lower case 1. Upper case 7 has through-hole 7A.
Columnar section 6A of push button 6 is inserted into through-hole 7A of upper case 7 and is supported movably in up-and-down directions. The upper part of columnar section 6A protrudes from through-hole 7A in an upward direction. An upper surface of flange 6B of push button 6 contacts a lower surface of upper case 7. When push button 6 is not activated, push button 6 has recess 6C slightly push upper projection 5B of elastic operation body 5. Elastic operation body 5 urges push button 6 towards upper case 7, thereby preventing push button 6 from wobbling.
An operation of push switch 501 will be described below. FIG. 8 is a cross-sectional view of push switch 501 which is activated.
When columnar section 6A of push button 6 protruding from through-hole 7A of upper case 7 is depressed, push button 6 moves downward while being supported by through-hole 7A. When push button 6 moves, a pressing force is applied to upper projection 5B of elastic operation body 5 inserted into recess 6C of push button 6. The force pressurizes and compresses elastic operation body 5 in its longitudinal direction provided from upper projection 5B to lower projection 5C, accordingly applying a pressure via elastic operation body 5 to the top of the dome shape of movable contact 4 contacting lower projection 5C. When the pressure exceeds a predetermined value, the top of the dome shape of movable contact 4 is elastically inverted to have the lower surface of movable contact 4 contact inner fixed contact 2A with a click feeling, and connects outer fixed contacts 3A and 3B with inner fixed contact 2A via movable contact 4, thereby turning on push switch 501.
When movable contact 4 contacts inner fixed contact 2A, push button can be further pushed with a further pressing force downwardly. In this situation, outer fixed contacts 3A and 3B are electrically connected with inner fixed contact 2A via movable contact 4, as shown in FIG. 8, and upper projection 5B and lower projection 5C of elastic operation body 5 are compressed further elastically.
When the pressing force applied to push button 6 is removed, the pressure applied to elastic operation body 5 is canceled. Upper projection 5B and lower projection 5C accordingly return to have their original shapes and push up push button 6. Then, movable contact 4 is inverted to return to have its original dome shape. Then, movable contact 4 is removed away from inner fixed contact 2A, as shown in FIG. 6, to electrically disconnect outer fixed contacts 3A and 3B from inner fixed contact 2A, thereby turning off push switch 501.
In order to allow push button 6 to be movable, narrow gap 501A must be provided between through-hole 7A of upper case 7 and columnar section 6A of push button 6. When push switch 501 is activated, the upper surface of flange 6B of push button 6 is removed from the lower surface of upper case 7, as shown in FIG. 8, and an inside of the recess of lower case 1 communicates with the outside of push switch 501 via gap 501A. This may allow dust existing the outside of switch 501 to enter into the recess of lower case 1 through gap 501A during the activation of push switch 501. The dust would prevent movable contact 4 from contacting the fixed contacts, thereby preventing push switch 501 from operating appropriately.